Reorder split names

Reorder split names in the YAML metadata to be aligned with the order in the loading script.
See: https://github.com/huggingface/datasets/issues/5258

README.md

@@ -42,12 +42,12 @@ dataset_info:
   - name: ja_sentence
     dtype: string
   splits:
-  - name: test
-    num_bytes: 493038
-    num_examples: 2120
   - name: train
     num_bytes: 4778409
     num_examples: 20000
+  - name: test
+    num_bytes: 493038
+    num_examples: 2120
   - name: validation
     num_bytes: 477964
     num_examples: 2051